The invention relates to a device and a method for continuous chemical vapour deposition under atmospheric pressure on substrates. The device is hereby based on a reaction chamber, along the open sides of which the substrates are guided, as a result of which the corresponding coatings can be effected on the side of the substrates which is orientated towards the chamber interior.
The production of thin layers made of gaseous starting materials (so-called precursors) is implemented with a large number of technical realisations. It is common to all methods that a gaseous precursor or a precursor brought into the gas phase is conducted into a reaction chamber, is decomposed there by the coupling in of energy and components of the gas are deposited on the parts to be coated. One of these methods is atmospheric pressure chemical vapour deposition (termed APCVD). It is characterised in that the precursor and the process chamber are almost at atmospheric pressure. An example of APCVD is APCVD epitaxy of silicon layers made of chlorosilanes. In this case the chlorosilane, normally mixed with hydrogen, is degraded in the reaction chamber at temperatures around 1000-1200° C. and silicon is deposited on a crystalline silicon substrate with the same crystal orientation. This process is used inter alia for solar cells which comprise thin, crystalline Si layers. In particular for this application case, silicon deposition reactors are required, which can deposit an approx. 10-20 μm thick Si layer very economically (under 30 C=/m2) and at a high throughput (>20 m2/h). The reactors corresponding to the state of the art cannot achieve these requirements because they a) have too little throughput (e.g. ASM Epsilon 3000:1m2/h) and b) use the silicon contained in the precursor only very incompletely (a few percent). A new development concerns the production of a high throughput reactor for chemical vapour deposition/epitaxy of silicon (Hurrle, S. Reber, N. Schillinger, J. Haase, J. G. Reichart, “High Throughput Continuous CVD Reactor for Silicon Deposition”, in Proc. 19th European Conference on Photovoltaic Energy Conversion (WIP—Munich, ETA—Florence 2004, p. 459). In addition to the deposition of silicon, also all other layers which can be deposited under atmospheric pressure are in principle thereby producible in this reactor.
The reactor embodies the following principle (see FIG. 1): 2 parallel rows of substrates 1, 1′ are moved into a pipe 2 through a gas lock. In the interior of the pipe there is a chamber 3 which is open on the left and on the right. These openings of the chamber are also termed subsequently “deposition zone”. One row of substrates respectively is moved past on an open side of the chamber, closes the opening and thereby seals the chamber volume relative to the pipe volume. The precursor is introduced into the chamber from the front (i.e. the side of the inlet gas lock) through a gas inlet 4 and is suctioned-off through a gas outlet 5 in the rear region of the chamber. A special feature of the deposition chamber is that, relative to the volume situated outside the chamber, a small low pressure is maintained. This prevents large quantities of process gas escaping from the chamber. At the above-mentioned temperatures, the precursor (here: SiHCl3/H2) is degraded and silicon is deposited principally on the continuously rearwardly-moving inner sides of the rows of substrates. The process gas mixture is preferably chosen such that the gas is completely depleted at the rear end of the chamber and no further deposition takes place. As a result, a deposition profile (i.e. a profile or a different deposition thickness) is produced naturally, which is however completely compensated for by the movement of the substrates. The substrates leave the unit at the rear end of the pipe again through a gas lock. A further feature of the reactor is that the substrates can be coated continuously at a uniform feed rate, i.e. a cycled operation which is complex to control is not required.
At the parts 6 of the chamber which are produced from graphite and also at other surfaces, undesired “parasitic” depositions are produced. These must be removed regularly in order that all the cross-sections are maintained and hence no disturbing flakes are formed. In addition to the chamber surfaces, for example also the gas inlet nozzle or the gas outlet opening is affected by parasitic depositions.
The described principle must scale-up in throughput to a plant suitable for the production of solar cells and also must optimise as far as possible the operating time of the plant, i.e. ensure an interruption-free permanent operation as far as possible. The present invention takes this requirement into account.